What makes some species invasive while others never thrive outside their native lands? In some cases, it may not be a characteristic of the invasive species itself but rather the microbes that hitch a ride along with it. Harlequin ladybugs (Harmonia axyridis), also known as the multi-colored Asian ladybug, were introduced to the U.S. and Europe from eastern Asia to help control agricultural pests like aphids. Now these insects are sometimes a nuisance themselves when they invade homes in the autumn looking for shelter to over-winter. More alarmingly, they outcompete native Coccinella ladybugs, who consume a variety of garden insect pests, leading to declines in native ladybug populations. How do they do it? Both types of ladybugs eat not only aphids, but also other each other. Harlequin ladybugs eat Coccinella eggs and larvae without ill effect, but when Coccinella ladybugs eat Harlequin eggs and larvae, they die. Researchers in Germany have found that Harlequin ladybugs carry a parasitic microsporidia that causes a lethal infection in any Coccinella that eats them. The microsporidia does not kill Harlequin ladybugs because the ladybugs produce an alkyloid chemical called harmonine that suppresses the parasite. The good news is that harmonine suppresses many types of pathogens, including those that cause malaria and tuberculosis in humans, making it a potential lead in developing new anti-infective drugs. Sadly this will not help natives like the nine-spotted ladybug. Once the most common ladybug in the Northeastern U.S., it has now almost disappeared from its historical range. The causes of its decline are unclear but may include infection with microsporidia acquired from invasive Harlequin ladybugs. Professor Andreas Vilcinskas, of the University of Giessen in Germany, says that microsporidia have been found in samples of Harlequin ladybugs from North America. He and his colleagues are currently analyzing DNA from the parasites to determine if they are the same species that are present in Harlequin from Germany. How native ladybug declines and loss of ladybug diversity will effect populations of plant-eating pest insects is something we may soon find out, like it or not. You can help scientists document changes in species distributions of ladybugs by visiting the Lost Ladybug Project, a citizen science project organized by Cornell University and 4-H.